Neisseria meningitidis is responsible for sepsis and meningitis worldwide. Currently no vaccine against serogroup B strains is available in the U.S. but a serogroup B vaccine recently was licensed in the European Union, Australia and Canada. This vaccine contains factor H binding protein (fHbp) as one of its principal antigens. The protein recruits complement factor H (fH) to the bacterial surface, which allows the bacteria to evade host immunity. Binding of fH to fHbp is specific for human fH and, as we recently discovered, for fH in a subset of rhesus macaques. Based on studies in transgenic mice that expressed human fH, binding of fH to the fHbp vaccine decreased protective antibody responses. Further, mutant fHbp vaccines with single amino acid substitutions that eliminated fH binding had increased protective antibody responses, compared to control fHbp vaccines that bound human fH. However, there can be differences in antibody repertoire between rodents and primates and there are many examples of promising vaccine candidates in mice that failed to elicit protection in humans. Therefore, immunogenicity studies in a nonhuman primate model where fH binds to the control fHbp vaccine, but not to the mutant vaccine, would provide compelling rationale for development of mutant fHbp vaccines for humans. In Aim 1, we will compare protective serum antibody responses of rhesus macaques immunized with a recombinant mutant fHbp vaccine that does not bind rhesus fH to those of a control fHbp vaccine that binds rhesus fH. In Aim 2, we will characterize the antibody repertoires to the two vaccines by generating recombinant anti-fHbp fragments (Fabs) from variable region genes isolated from B cells of vaccinated animals. The locations of the epitopes recognized by the Fabs will be defined using a mutant fHbp library in yeast. The Fabs also will be tested for inhibition of binding of fH to fHbp, which is important for anti-fHbp antibody protective activity. However, since Fabs do not engage C1q or activate complement, in Aim 3, we will investigate anti-fHbp antibody protective activity in relation to epitope location by constructing chimeric IgG antibodies containing combining sites from rhesus-derived antibodies and effector portions of human IgG1 antibodies. Using human complement, the chimeric antibodies will be tested for activation of C3b deposition on the bacterial surface and bactericidal activity, and passive protection against bacteremia in human fH transgenic infant rats. Collectively, the data will provide insight into mechanisms responsible for increased immunogenicity of a prototype fHbp vaccine engineered to have decreased fH binding. The results will be directly applicable to development of more effective meningococcal serogroup B vaccines for humans. The mutant vaccines also may be safer, because of less risk of inducing auto-reactive anti-fH antibodies.